var detectCycle = function(head) {//*
    let fast=slow=cur=head
    while(fast&&fast.next){//这个条件要搞清楚
        fast=fast.next.next
        slow=slow.next
        if(slow===fast){
            while(slow!==cur){
                slow=slow.next
                cur=cur.next
            }
            return slow
        }
    }
    return null
};